Add prefix_multi option for riak kv backend
Add prefix_multi option for riak kv backend

Cf. Introduce `riak_kv_multi_prefix_backend` by kuenishi · Pull Request #1082
    https://github.com/basho/riak_kv/pull/1082

dragonfax/riak_cs,dragonfax/riak_cs,basho/riak_cs,basho/riak_cs,basho/riak_cs,dragonfax/riak_cs,basho/riak_cs,GabrielNicolasAvellaneda/riak_cs,dragonfax/riak_cs,GabrielNicolasAvellaneda/riak_cs,GabrielNicolasAvellaneda/riak_cs,GabrielNicolasAvellaneda/riak_cs